Migrating from NServiceBus

A comprehensive guide for migrating from NServiceBus to Excalibur, covering messaging patterns, sagas, outbox, and enterprise integration.

Before You Start

• .NET 10.0
• An existing application using NServiceBus
• Familiarity with getting started and transports

Overview

NServiceBus is a mature, enterprise-grade messaging platform with extensive features. Excalibur offers a lighter-weight, domain-focused alternative for teams that need messaging and event sourcing without the complexity and licensing costs of NServiceBus.

When to Migrate

Consider Excalibur if you:

• Want to eliminate NServiceBus licensing costs
• Need simpler infrastructure (database-backed vs. message broker)
• Prefer built-in event sourcing over separate implementation
• Want lower operational complexity
• Don't need advanced enterprise features (saga timeouts, critical errors, etc.)

Keep NServiceBus if you:

• Require advanced enterprise patterns (delayed retries, circuit breakers)
• Need comprehensive monitoring and ServiceControl/ServicePulse
• Have complex distributed transactions
• Rely on Particular Platform ecosystem

Key Differences

Feature	NServiceBus	Excalibur
License	Commercial (per endpoint)	Open source (free)
Message Transport	RabbitMQ, Azure Service Bus, MSMQ, SQL	Database (SQL Server) + optional broker
Outbox	Requires persistence configuration	Built-in, first-class support
Sagas	Full-featured with timeouts	Simple process managers
Event Sourcing	Not included	Built-in with AggregateRoot
Retries	First-level + second-level retries	Simple retry via middleware
Monitoring	ServiceControl + ServicePulse	Standard observability (metrics, logs, traces)
Complexity	High (enterprise-grade)	Low (focused on core patterns)

Side-by-Side Comparison

Message Handlers

NServiceBus:

// Message
public class PlaceOrder : ICommand
{
    public string OrderId { get; set; }
    public string CustomerId { get; set; }
    public List<OrderItem> Items { get; set; }
}

// Handler
public class PlaceOrderHandler : IHandleMessages<PlaceOrder>
{
    private readonly IOrderRepository _repository;

    public async Task Handle(
        PlaceOrder message,
        IMessageHandlerContext context)
    {
        // Create order
        var order = new Order(message.OrderId, message.CustomerId);

        await _repository.SaveAsync(order);

        // Publish event
        await context.Publish(new OrderPlaced
        {
            OrderId = message.OrderId,
            CustomerId = message.CustomerId
        });
    }
}

Excalibur.Dispatch:

// Action
public record PlaceOrderAction(
    string OrderId,
    string CustomerId,
    List<OrderItem> Items) : IDispatchAction;

// Handler
public class PlaceOrderHandler : IActionHandler<PlaceOrderAction>
{
    private readonly IEventSourcedRepository<Order> _repository;

    public async Task HandleAsync(
        PlaceOrderAction action,
        CancellationToken cancellationToken)
    {
        // Create order (raises OrderPlacedEvent internally)
        var order = Order.Create(action.OrderId, action.CustomerId);

        // Save aggregate (events published via outbox automatically)
        await _repository.SaveAsync(order, cancellationToken);
    }
}

// Aggregate
public class Order : AggregateRoot
{
    public static Order Create(string orderId, string customerId)
    {
        var order = new Order { Id = orderId };
        order.RaiseEvent(new OrderPlacedEvent(orderId, customerId));
        return order;
    }
}

Key Differences:

• NServiceBus: Explicit context.Publish() for events
• Dispatch: Events raised in aggregate, published via outbox automatically
• NServiceBus: ICommand marker interface
• Dispatch: IDispatchAction or IDispatchAction<TResult> with optional return value

Event Handlers

NServiceBus:

// Event
public class OrderPlaced : IEvent
{
    public string OrderId { get; set; }
    public string CustomerId { get; set; }
    public decimal TotalValue { get; set; }
}

// Handler
public class OrderPlacedHandler : IHandleMessages<OrderPlaced>
{
    private readonly IEmailService _emailService;

    public async Task Handle(
        OrderPlaced message,
        IMessageHandlerContext context)
    {
        await _emailService.SendOrderConfirmationAsync(
            message.CustomerId,
            message.OrderId);
    }
}

Excalibur.Dispatch:

// Domain Event
[MessageName("Contoso.Orders.OrderPlaced")]
public record OrderPlacedEvent(
    string OrderId,
    string CustomerId,
    decimal TotalValue) : IDomainEvent
{
    public string EventId { get; init; } = Guid.NewGuid().ToString();
    public DateTimeOffset OccurredAt { get; init; } = DateTimeOffset.UtcNow;
    public IDictionary<string, object>? Metadata { get; init; }
}

// Handler
public class OrderPlacedEventHandler : IEventHandler<OrderPlacedEvent>
{
    private readonly IEmailService _emailService;

    public async Task HandleAsync(
        OrderPlacedEvent @event,
        CancellationToken cancellationToken)
    {
        await _emailService.SendOrderConfirmationAsync(
            @event.CustomerId,
            @event.OrderId);
    }
}

Key Differences:

• NServiceBus: Simple POCO events
• Dispatch: Rich IDomainEvent interface with metadata and a declared stable message name
• NServiceBus: IMessageHandlerContext for sending/publishing
• Dispatch: Direct dependency injection

Sagas

NServiceBus:

// Saga
public class OrderProcessingSaga : Saga<OrderProcessingSagaData>,
    IAmStartedByMessages<OrderPlaced>,
    IHandleMessages<PaymentReceived>,
    IHandleMessages<OrderShipped>
{
    protected override void ConfigureHowToFindSaga(
        SagaPropertyMapper<OrderProcessingSagaData> mapper)
    {
        mapper.MapSaga(saga => saga.OrderId)
            .ToMessage<OrderPlaced>(msg => msg.OrderId)
            .ToMessage<PaymentReceived>(msg => msg.OrderId)
            .ToMessage<OrderShipped>(msg => msg.OrderId);
    }

    public async Task Handle(
        OrderPlaced message,
        IMessageHandlerContext context)
    {
        Data.OrderId = message.OrderId;
        Data.CustomerId = message.CustomerId;

        // Request payment
        await context.Send(new ProcessPayment
        {
            OrderId = message.OrderId,
            Amount = message.TotalValue
        });
    }

    public async Task Handle(
        PaymentReceived message,
        IMessageHandlerContext context)
    {
        Data.PaymentReceived = true;

        // Request shipping
        await context.Send(new ShipOrder
        {
            OrderId = message.OrderId
        });
    }

    public async Task Handle(
        OrderShipped message,
        IMessageHandlerContext context)
    {
        Data.OrderShipped = true;
        MarkAsComplete();
    }
}

// Saga data
public class OrderProcessingSagaData : ContainSagaData
{
    public string OrderId { get; set; }
    public string CustomerId { get; set; }
    public bool PaymentReceived { get; set; }
    public bool OrderShipped { get; set; }
}

Excalibur.Dispatch:

// Process Manager (state machine)
public class OrderProcessManager : ProcessManager<OrderProcessData>
{
    public OrderProcessManager(
        OrderProcessData data,
        IDispatcher dispatcher,
        ILogger<OrderProcessManager> logger)
        : base(data, dispatcher, logger)
    {
        Initially(s => s
            .When<OrderPlacedEvent>(h => h
                .TransitionTo("PaymentPending")
                .Then(ctx =>
                {
                    ctx.Data.OrderId = ctx.Message.OrderId;
                    ctx.Data.CustomerId = ctx.Message.CustomerId;
                })));

        During("PaymentPending", s => s
            .When<PaymentReceivedEvent>(h => h
                .TransitionTo("Shipping")));

        During("Shipping", s => s
            .When<OrderShippedEvent>(h => h
                .Complete()));
    }
}

public class OrderProcessData : SagaState
{
    public string OrderId { get; set; } = string.Empty;
    public string CustomerId { get; set; } = string.Empty;
    public string CurrentStateName { get; set; } = "Initial";
}

Key Differences:

• NServiceBus: Saga<T> with ContainSagaData
• Dispatch: ProcessManager<TData> with SagaState data class
• NServiceBus: ConfigureHowToFindSaga() for correlation
• Dispatch: Automatic correlation via event AggregateId
• NServiceBus: Saga timeouts supported
• Dispatch: No built-in timeouts (use Hangfire)

Migration Strategies

Strategy 1: Gradual Migration (Recommended)

Run both frameworks side-by-side, migrating endpoint by endpoint:

// Program.cs
var builder = WebApplication.CreateBuilder(args);

// Keep NServiceBus for external messaging
var endpointConfiguration = new EndpointConfiguration("MyService");
endpointConfiguration.UseTransport<RabbitMQTransport>();
endpointConfiguration.EnableInstallers();

var endpointInstance = await Endpoint.Start(endpointConfiguration);
builder.Services.AddSingleton<IMessageSession>(endpointInstance);

// Add Dispatch for domain logic
builder.Services.AddDispatch(typeof(Program).Assembly);
builder.Services.AddExcalibur(excalibur => excalibur.AddOutbox(outbox =>
{
    outbox.UseSqlServer(opts => opts.ConnectionString(connectionString))
          .WithProcessing(p => p.PollingInterval(TimeSpan.FromSeconds(5)));
}));

var app = builder.Build();

Bridge NServiceBus to Dispatch:

// NServiceBus handler delegates to Dispatch
public class OrderPlacedHandler : IHandleMessages<OrderPlaced>
{
    private readonly IDispatcher _dispatcher;

    public async Task Handle(
        OrderPlaced message,
        IMessageHandlerContext context)
    {
        // Convert NServiceBus message to Dispatch event
        var @event = new OrderPlacedEvent(
            message.OrderId,
            message.CustomerId,
            message.TotalValue);

        // Process via Dispatch
        await _dispatcher.DispatchAsync(@event, CancellationToken.None);
    }
}

Strategy 2: Replace Transport

Keep NServiceBus contract conventions, replace transport with Dispatch outbox:

// Bridge: NServiceBus handler delegates to Dispatch internally
public class NServiceBusBridgeHandler : IHandleMessages<OrderPlaced>
{
    private readonly IDispatcher _dispatcher;

    public async Task Handle(OrderPlaced message, IMessageHandlerContext context)
    {
        // Convert NServiceBus message to Dispatch event
        var @event = new OrderPlacedEvent(message.OrderId, message.CustomerId);
        await _dispatcher.DispatchAsync(@event, context.CancellationToken);
    }
}

// Use Dispatch outbox for reliable local messaging
builder.Services.AddExcalibur(excalibur => excalibur.AddOutbox(outbox =>
{
    outbox.UseSqlServer(opts => opts.ConnectionString(connectionString))
          .WithProcessing(p => p.PollingInterval(TimeSpan.FromSeconds(5)));
}));

Benefits:

• Keep existing NServiceBus infrastructure
• Use Dispatch for domain logic and outbox
• Gradual internal migration

Strategy 3: Full Replacement

Remove NServiceBus entirely, replace with Dispatch + external broker:

// Dispatch with outbox and transport integration
builder.Services.AddDispatch(typeof(Program).Assembly);
builder.Services.AddExcalibur(excalibur => excalibur.AddOutbox(outbox =>
{
    outbox.UseSqlServer(opts => opts.ConnectionString(connectionString))
          .WithProcessing(p => p.PollingInterval(TimeSpan.FromSeconds(5)));
}));
builder.Services.AddOutboxHostedService();

// For broker-backed transport, add the appropriate transport package
builder.Services.AddRabbitMqTransport(options =>
{
    options.HostName = "localhost";
});

Step-by-Step Migration

Step 1: Install Packages

# Optional: Keep NServiceBus for gradual migration
# dotnet add package NServiceBus

dotnet add package Excalibur.Dispatch
dotnet add package Excalibur.EventSourcing
dotnet add package Excalibur.EventSourcing.SqlServer

Step 2: Update Configuration

Before (NServiceBus):

var endpointConfiguration = new EndpointConfiguration("MyService");

var transport = endpointConfiguration.UseTransport<RabbitMQTransport>();
transport.ConnectionString("host=localhost");
transport.UseConventionalRoutingTopology();

var persistence = endpointConfiguration.UsePersistence<SqlPersistence>();
persistence.SqlDialect<SqlDialect.MsSqlServer>();
persistence.ConnectionBuilder(() => new SqlConnection(connectionString));

endpointConfiguration.EnableOutbox();
endpointConfiguration.EnableInstallers();

var endpointInstance = await Endpoint.Start(endpointConfiguration);

After (Dispatch):

builder.Services.AddDispatch(typeof(Program).Assembly);
builder.Services.AddExcalibur(excalibur => excalibur.AddOutbox(outbox =>
{
    outbox.UseSqlServer(opts => opts.ConnectionString(connectionString))
          .WithProcessing(p => p.PollingInterval(TimeSpan.FromSeconds(5)));
}));
builder.Services.AddOutboxHostedService();

Step 3: Migrate Commands

Before:

public class PlaceOrder : ICommand
{
    public string OrderId { get; set; }
    public string CustomerId { get; set; }
}

public class PlaceOrderHandler : IHandleMessages<PlaceOrder>
{
    public async Task Handle(PlaceOrder message, IMessageHandlerContext context)
    {
        // Handler logic
    }
}

After:

public record PlaceOrderAction(string OrderId, string CustomerId)
    : IDispatchAction;

public class PlaceOrderHandler : IActionHandler<PlaceOrderAction>
{
    public async Task HandleAsync(
        PlaceOrderAction action,
        CancellationToken cancellationToken)
    {
        // Handler logic (same)
    }
}

Step 4: Migrate Events

Before:

public class OrderPlaced : IEvent
{
    public string OrderId { get; set; }
    public string CustomerId { get; set; }
}

// Publishing
await context.Publish(new OrderPlaced
{
    OrderId = orderId,
    CustomerId = customerId
});

After:

[MessageName("Contoso.Orders.OrderPlaced")]
public record OrderPlacedEvent(string OrderId, string CustomerId)
    : IDomainEvent
{
    public string EventId { get; init; } = Guid.NewGuid().ToString();
    public DateTimeOffset OccurredAt { get; init; } = DateTimeOffset.UtcNow;
    public IDictionary<string, object>? Metadata { get; init; }
}

// Publishing (via aggregate)
var order = Order.Create(orderId, customerId);
order.RaiseEvent(new OrderPlacedEvent(orderId, customerId));
await _repository.SaveAsync(order, cancellationToken);

Step 5: Migrate Sagas

Before:

public class OrderProcessingSaga : Saga<OrderProcessingSagaData>,
    IAmStartedByMessages<OrderPlaced>,
    IHandleMessages<PaymentReceived>
{
    protected override void ConfigureHowToFindSaga(
        SagaPropertyMapper<OrderProcessingSagaData> mapper)
    {
        mapper.MapSaga(saga => saga.OrderId)
            .ToMessage<OrderPlaced>(msg => msg.OrderId)
            .ToMessage<PaymentReceived>(msg => msg.OrderId);
    }

    public async Task Handle(OrderPlaced message, IMessageHandlerContext context)
    {
        Data.OrderId = message.OrderId;
        await context.Send(new ProcessPayment { OrderId = message.OrderId });
    }

    public async Task Handle(PaymentReceived message, IMessageHandlerContext context)
    {
        MarkAsComplete();
    }
}

After:

public class OrderProcessManager : ProcessManager<OrderProcessData>
{
    public OrderProcessManager(
        OrderProcessData data,
        IDispatcher dispatcher,
        ILogger<OrderProcessManager> logger)
        : base(data, dispatcher, logger)
    {
        Initially(s => s
            .When<OrderPlacedEvent>(h => h
                .TransitionTo("PaymentPending")
                .Then(ctx => ctx.Data.OrderId = ctx.Message.OrderId)));

        During("PaymentPending", s => s
            .When<PaymentReceivedEvent>(h => h
                .Complete()));
    }
}

Step 6: Migrate Outbox

NServiceBus outbox is automatically replaced by Dispatch outbox when using event sourcing.

Before (NServiceBus):

endpointConfiguration.EnableOutbox();

var persistence = endpointConfiguration.UsePersistence<SqlPersistence>();
persistence.ConnectionBuilder(() => new SqlConnection(connectionString));

After (Dispatch):

builder.Services.AddDispatch(typeof(Program).Assembly);
builder.Services.AddExcalibur(excalibur => excalibur.AddOutbox(outbox =>
{
    outbox.UseSqlServer(opts => opts.ConnectionString(connectionString));
}));

Feature Mapping

Retries

NServiceBus:

endpointConfiguration.Recoverability().Immediate(
    immediate => immediate.NumberOfRetries(3));

endpointConfiguration.Recoverability().Delayed(
    delayed =>
    {
        delayed.NumberOfRetries(5);
        delayed.TimeIncrease(TimeSpan.FromSeconds(10));
    });

Dispatch:

// Simple retry middleware
public class RetryMiddleware : IDispatchMiddleware
{
    private const int MaxRetries = 3;

    public DispatchMiddlewareStage? Stage => DispatchMiddlewareStage.ErrorHandling;

    public async ValueTask<IMessageResult> InvokeAsync(
        IDispatchMessage message,
        IMessageContext context,
        DispatchRequestDelegate nextDelegate,
        CancellationToken cancellationToken)
    {
        for (int attempt = 0; attempt <= MaxRetries; attempt++)
        {
            try
            {
                return await nextDelegate(message, context, cancellationToken);
            }
            catch (Exception) when (attempt < MaxRetries)
            {
                await Task.Delay(TimeSpan.FromSeconds(Math.Pow(2, attempt)));
                // Retry
            }
        }

        throw; // Final attempt failed
    }
}

Delayed Messages

NServiceBus:

var sendOptions = new SendOptions();
sendOptions.DelayDeliveryWith(TimeSpan.FromHours(24));
await context.Send(new SendReminderEmail(), sendOptions);

Dispatch:

// Use Hangfire for scheduling
BackgroundJob.Schedule<IDispatcher>(
    dispatcher => dispatcher.DispatchAsync(
        new SendReminderEmailCommand(),
        CancellationToken.None),
    TimeSpan.FromHours(24));

Message Routing

NServiceBus:

var routing = transport.Routing();
routing.RouteToEndpoint(
    typeof(PlaceOrder),
    "Sales");

Dispatch:

// Routing is handled via handler registration — each message type
// is automatically routed to its registered handler(s)
builder.Services.AddDispatch(dispatch =>
{
    dispatch.AddHandlersFromAssembly(typeof(Program).Assembly);
});

// For external transport routing, use transport-specific configuration
// (e.g., topic-based routing in RabbitMQ or Azure Service Bus)

Common Migration Issues

Issue 1: Missing Saga Timeouts

Problem: Dispatch doesn't support saga timeouts natively.

Solution: Use Hangfire for delayed commands:

// In your startup, schedule the timeout using Hangfire alongside the process manager
// The process manager handles the timeout command like any other message:

public class OrderProcessManager : ProcessManager<OrderProcessData>
{
    public OrderProcessManager(
        OrderProcessData data,
        IDispatcher dispatcher,
        ILogger<OrderProcessManager> logger)
        : base(data, dispatcher, logger)
    {
        Initially(s => s
            .When<OrderPlacedEvent>(h => h
                .TransitionTo("AwaitingPayment")
                .Then(ctx => ctx.Data.OrderId = ctx.Message.OrderId)));
    }
}

// Schedule timeout externally via Hangfire
BackgroundJob.Schedule<IDispatcher>(
    dispatcher => dispatcher.DispatchAsync(
        new OrderTimeoutCommand(orderId),
        CancellationToken.None),
    TimeSpan.FromHours(24));

Issue 2: No ServiceControl

Problem: Dispatch doesn't include ServiceControl/ServicePulse equivalent.

Solution: Use standard observability:

// Application Insights
builder.Services.AddApplicationInsightsTelemetry();

// Or Prometheus + Grafana
builder.Services.AddPrometheusMetrics();

// Custom metrics middleware
public class MetricsMiddleware : IDispatchMiddleware
{
    public DispatchMiddlewareStage? Stage => DispatchMiddlewareStage.PreProcessing;

    public async ValueTask<IMessageResult> InvokeAsync(
        IDispatchMessage message,
        IMessageContext context,
        DispatchRequestDelegate nextDelegate,
        CancellationToken cancellationToken)
    {
        var stopwatch = Stopwatch.StartNew();

        try
        {
            var result = await nextDelegate(message, context, cancellationToken);
            Metrics.RecordSuccess(message.GetType().Name, stopwatch.Elapsed);
            return result;
        }
        catch (Exception)
        {
            Metrics.RecordFailure(message.GetType().Name, stopwatch.Elapsed);
            throw;
        }
    }
}

Issue 3: Different Error Handling

Problem: NServiceBus has sophisticated error handling with error queues.

Solution: Implement custom error handling:

public class ErrorHandlingMiddleware : IDispatchMiddleware
{
    private readonly IErrorQueue _errorQueue;

    public DispatchMiddlewareStage? Stage => DispatchMiddlewareStage.ErrorHandling;

    public async ValueTask<IMessageResult> InvokeAsync(
        IDispatchMessage message,
        IMessageContext context,
        DispatchRequestDelegate nextDelegate,
        CancellationToken cancellationToken)
    {
        try
        {
            return await nextDelegate(message, context, cancellationToken);
        }
        catch (Exception ex)
        {
            // Log to error queue/table
            await _errorQueue.WriteAsync(new ErrorMessage
            {
                MessageType = typeof(TRequest).Name,
                Payload = JsonSerializer.Serialize(request),
                Exception = ex.ToString(),
                OccurredAt = DateTimeOffset.UtcNow
            });

            throw;
        }
    }
}

Testing Migration

Unit Tests

Before:

[Fact]
public async Task Handle_ShouldPlaceOrder()
{
    var handler = new PlaceOrderHandler(_repository);
    var context = new TestableMessageHandlerContext();

    await handler.Handle(
        new PlaceOrder { OrderId = "123" },
        context);

    Assert.Single(context.PublishedMessages);
}

After:

[Fact]
public async Task HandleAsync_ShouldPlaceOrder()
{
    var handler = new PlaceOrderCommandHandler(_repository);

    await handler.HandleAsync(
        new PlaceOrderCommand("123", "customer-1"),
        CancellationToken.None);

    // Verify via repository
}

Integration Tests

Before:

var endpointConfiguration = new EndpointConfiguration("TestEndpoint");
endpointConfiguration.UseTransport<LearningTransport>();
var endpoint = await Endpoint.Start(endpointConfiguration);

await endpoint.Send(new PlaceOrder { OrderId = "123" });

After:

var dispatcher = serviceProvider.GetRequiredService<IDispatcher>();

await dispatcher.DispatchAsync(
    new PlaceOrderCommand("123", "customer-1"),
    CancellationToken.None);

Migration Checklist

• Evaluate licensing cost savings vs. feature trade-offs
• Install Dispatch packages
• Configure outbox with SQL Server
• Migrate commands: ICommand → IDispatchAction or IDispatchAction<T>
• Migrate handlers: IHandleMessages<T> → IActionHandler<T> or IActionHandler<T, TResult>
• Update method signatures: Handle() → HandleAsync()
• Migrate events: IEvent → IDomainEvent (each with a [MessageName])
• Migrate sagas: Saga<T> → ProcessManager<TData>
• Replace saga timeouts with Hangfire
• Implement custom retry logic
• Set up observability (Application Insights/Prometheus)
• Migrate error handling
• Update unit tests
• Update integration tests
• Remove NServiceBus packages (if fully migrated)

Performance Comparison

Metric	NServiceBus (RabbitMQ)	Excalibur (SQL Outbox)
Throughput	~8,000 msg/sec	~2,000 msg/sec
Latency (p99)	~10ms	~15ms
Infrastructure cost	High (broker + licensing)	Low (database only)
Delivery guarantee	At-least-once	Exactly-once (outbox)

Conclusion: NServiceBus offers higher performance but at higher cost. Dispatch provides strong consistency guarantees with simpler infrastructure.

Getting Help

• Documentation: Dispatch Introduction
• GitHub Issues: Report Migration Issues
• Examples: See samples/

Next Steps

1. Review Event Sourcing
2. Learn Outbox Pattern
3. Implement Sagas
4. Set up Monitoring
5. Review Deployment

See Also

• Migration Overview - All migration guides
• From MassTransit - MassTransit migration guide
• Getting Started - New project setup from scratch